Control of the acid-base balance in the human body is maintained by intricate renal and pulmonary mechanisms. A disturbance in this balance is generally accompanied by changes in the electrolyte composition of the blood. Therefore, several analyses are necessary to ascertain the acid-base status. One important analysis performed in the clinical laboratory is the determination of the concentration of carbon dioxide in the blood.
Carbon dioxide dissolved in blood is in equilibrium between the interior of red blood cells and the plasma and also within the plasma. It undergoes the following reaction: EQU CO.sub.2 +H.sub.2 O.revreaction.H.sub.2 CO.sub.3 .revreaction.HCO.sub.3 --+H.sup.+ .revreaction.2H.sup.+ +CO.sub.3 .dbd.
The interrelationship between total CO.sub.2, HCO.sub.3 --, dissolved CO.sub.2 and pH (not taking into account the insignificant amounts of CO.sub.3 .dbd. and carbamino compounds) is described in the literature (see Fundamentals of Clinical Chemistry, Norbert W. Tietz, ed., second edition, W. B. Saunders, Co., Philadelphia, Pa., p. 893, 1976). With the aid of the Henderson-Hasselbach equation described in the literature, one can calculate pH, pCO.sub.2, total CO.sub.2 and HCO.sub.3 -- knowing any two of these. pCO.sub.3 is the partial pressure of CO.sub.2 gas in a hypothetical gas phase with which the blood would be in equilibrium.
Potentiometric determination of the total CO.sub.2 has been performed using a carbonate ion-selective electrode. This requires the measurement of the pH of the sample to be tested or the fixing of the pH of the sample by the addition of a buffered solution prior to testing, such as described in U.S. Pat. No. 4,131,428. This procedure, however, requires a separate step in addition to the measurement by the ion-selective electrode. Moreover, the measurement of CO.sub.2 by carbonate ion-selective electrodes is further adversely affected by interferences by such ions as gentisate, salicylate and p-aminosalicylate. Therefore, an ion-selective electrode for the determination of CO.sub.2 in a liquid sample which does not require an additional step and which has a minimum of interference from gentisate, salicylate and p-aminosalicylate has been sought in the art.
Significant advances relating to ion-selective electrodes, and particularly, electrodes for the determination of CO.sub.2 are described in U.S. Pat. Nos. 4,214,968 (issued July 29, 1980 to Battaglia et al); 4,272,328 (issued June 9, 1981 to Kim et al) and 4,303,408 (issued Dec. 1, 1981 to Kim et al). In the first reference, improved dry-operative ion-selective electrodes are described. In the second reference, an ionophore-containing membrane layer is positioned between an electrolyte zone and an adjacent buffer zone containing a buffer sufficient to provide a pH in the range of from about 7.5 to about 9.5 when wetted with liquid. The third patent relates to the use of an interferent-removing zone in ion-selective electrodes to reduce the undesirable effects of salicylate and other interferents.
The search continues, however, for ion-selective electrodes which have even better precision in CO.sub.2 measurements and which exhibit reduced sensitivity to potential interferents, such as salicylate, particularly at high interferent levels in small children.
Another problem that can arise in using planar dry-operative ion-selective electrodes is that the outermost layer of the electrode, at least for certain assays, may be so hydrophobic, due to the chemistry, that drops of liquid to be assayed resist contact with the layer. That is, electrodes analyzing for CO.sub.2 and for other potentiometric ions advantageously feature a buffer overcoat or an emulsion overcoat. A useful buffer overcoat for CO.sub.2 electrodes is described, e.g., in the aforesaid U.S. Pat. No. 4,272,328. This overcoat has a high contact angle with aqueous liquids such as serum in view of its chemical components, and particularly the polymeric binder. It is nevertheless advantageous because it provides a buffer needed to convert CO.sub.2 to its ionic forms and acts to remove certain interferents.
Examples of useful emulsion overcoats can be found in aforesaid U.S. Pat. No. 4,303,408. These overcoats also tend to have a high surface contact angle with aqueous liquids, and yet are advantageous because they help to remove interferents. Alternatively, the emulsion overcoat is often conveniently combined with the aforementioned buffer overcoat, again producing a high contact angle.
When the aqueous sample drops resist contact with the electrode, the liquid tends to move up the side of the tip of the metering device or pipette, a condition known as "perfusion." Not only does this prevent proper spotting of the electrode at this time, it also interferes with the metering of successive drops. The problem then has been, prior to this invention, to find a method for reducing the contact angle of the liquid with the outermost electrode layer, particularly when such outermost layer is one of the aforesaid overcoats.
It is known, of course, that surfactants are commonly added to liquids to reduce their contact angle. However, optimum clinical analyses procedures are those that require no additional liquids to be added to the patient sample prior to analysis. If the surfactant is added to the web, it has not been clear which surfactant, if any, will outperform the others. Furthermore, if large amounts of surfactant are needed, there has been the concern that such large amounts may in themselves act as an interferent to the web chemistry. That is, although surfactants are commonly added to electrode layers as an aid to the coating of that layer, these have generally been added in amounts that are so small, e.g., 0.15 g/m.sup.2, that (a) they are not present in amounts that could be interferents, and (b) are ineffective to overcome the hydrophobic nature of the layer.